Mod-Con boiler with DHW ?

Discussion in 'HVAC Heating & Cooling' started by mc_1_2_3, May 29, 2008.

  1. mc_1_2_3

    mc_1_2_3 New Member

    Messages:
    23
    Location:
    Western Pennsylvania
    Hello. I am in the process of building a new house. Ranch style, radiant floor heat in basement and main floor. I have done alot of research on modulating condensing boilers and plan to purchase one. I'm leaning towards the Triangle Tube Prestige with either their indirect tank or a SuperStor. The heat loss/load calculation shows 63,000 btu. So, a Prestige at 100,000 btu would be plenty, and I'm sure enough for the added load of the indirect tank. My question is whether I would be better off with a smaller boiler from a different manufacturer. Would an 80,000 btu take care of the house load and the indirect hot water tank?

    Thanks,

    Mark
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,985
    Location:
    New England
    Most systems treat the DHW as a priority zone, and direct all heat to it when needed, so adding extra for that purpose is somewhat counterproductive unless it is something like a commercial spa or someplace where continuous hot water for showers, etc. is required. A typical WH has in the order of 30-50Kbtu. Heat to the house while it is heating the DHW tank has never been an issue, at least for my system - it just doesn't need to be on that long dedicated to that priority zone.

    An 80K with modulation would put it in the high efficiency mode most of the heating season and, depending on the size of the WH tank you choose, likely provide all of the hot water you need. Extra for the sake of extra is a waste in both initial cost and running costs. The house doesn't automatically become an icecube, plus, except in rare places, does the house sit for days on end at the design load, so max heat is rarely called for. So, instead of keeping it 72, it falls a degree for each point below the design load.
  3. BigLou

    BigLou New Member

    Messages:
    138
    I agree with jim 100% 100,000 btu sounds way to big. If you want more hot water just get a bigger tank.

    Lou
  4. mc_1_2_3

    mc_1_2_3 New Member

    Messages:
    23
    Location:
    Western Pennsylvania
    Thanks for the info. That's what I figured the answer would be. Now, just have decide which boiler. Have heard good and bad about many, just don't know what to believe. Any suggestions?

    Thanks,

    Mark
  5. Danish-joe

    Danish-joe New Member

    Messages:
    2
    Whatever you do take the sales man's opinion with a large grain of salt. They are biased & I have seen salesmen lead (or try to lead) me and executives so far astray it is unbelievable.

    Joe Jensen
    boilers Chillers
  6. mc_1_2_3

    mc_1_2_3 New Member

    Messages:
    23
    Location:
    Western Pennsylvania
    More questions, variable speed injection, mixing valves....

    Hello again. I will be using a properly sized condensing boiler with an indirect hot water tank. Now my issue involves the different temperature requirements of two different radiant applications. In a house I built 9 years ago, I just used a Slant Fin direct-vent cast iron boiler with two manual mixing valves to mix down the temperature for the basement concrete slab and the under-subfloor heat. No outdoor reset, just two thermostats. Did run the boiler at only 160 degrees, always had plenty of heat.

    With my new system, I want to take full advantage of outdoor reset and lowest possible boiler temperatures. With manual mixing valves, the temp in the loops won't go down, unless of course the boiler temp goes below the setting of the mixing valve. To do it correctly, I find that I will need a mixing solution that will also take advantage of outdoor reset. As I see it, the best way is to have two seperate injection pumps or two controllable mixing valves for the two different temperatures required. Am I correct? Any other suggestions? I am finding that the controls for dual injection systems can be quite expensive, 30 to 50 percent the cost of the boiler. Many condensing boilers have outdoor reset and DHW priority built-in. What I don't find is variable injection control. Any boilers have that?

    Thanks for any help,
    Mark
    Last edited: Jun 10, 2008
  7. Bob Walsh

    Bob Walsh New Member

    Messages:
    1
    Location:
    Manhattan
    condensing boilers

    Most condensing boiler manufacturers limit their boilers to operating with a 30% glycol content to avoid the boilers flashing the fluid to steam due to glycol’s inefficiency in heat transfer ability. Although a heat exchanger is an added expense, the cost of a condensing boiler and heat exchanger is frequently recovered because of the boiler efficiency at a high steady-state efficiency.

    High-efficiency boilers operating with 30% or less glycol content can be piped as secondaries to a primary system. This allows the coldest fluid to enter the boilers equally. Remember, condensing boilers love to see as low an entering fluid temperature as possible.


    ________________
    Greentech are specialists in a range of Condensing-Boilers Services.
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